1. Field of the Invention
The present invention relates to a condenser microphone capable of blocking entrance of electromagnetic waves to a microphone case and noise generation.
2. Description of the Related Art
An acoustic signal converted in a condenser microphone unit is output from a condenser microphone to the exterior thereof through a balanced shielded cable. The condenser microphone is detachably connected with the balanced shielded cable via a three-pin microphone connector. The connector is generally specified in EIAJ RC-5236 “Latch Lock Type Round Connector for Audio Equipment” (See “EIAJ RC-5236 Latch Lock Type Round Connector for Audio Equipment according to the Standards of the Japan Electrical Manufacturers' Association”).
FIG. 6 illustrates an example condenser microphone including such a specified connector (hereinafter referred to as “microphone connector”). Technical ideas of the present invention can be applied to the condenser microphone shown in FIG. 6.
An example conventional condenser microphone will be described with reference to FIG. 6. As shown in FIG. 6, a microphone unit 2 is included in an inner front end of a cylindrical microphone case 1 (the lower end in FIG. 6). The microphone unit 2 outputs electrical acoustic signals in response to sound waves.
A circuit board 3 is built in the microphone case 1. The acoustic signals are impedance-converted by an impedance converter that is included in the circuit board 3. Furthermore, signal processing such as amplification processing is performed. The specified microphone connector 4 is included in the rear end of the microphone case 1 (the upper end in FIG. 6).
The microphone connector 4 is a male connector which includes a pin insert 5 that is housed in a cylindrical connector sleeve 6 and is fixed to the connector sleeve 6 with a screw 9.
The pin insert 5 penetrates a base 7 that is composed of an insulating material, such as thermoplastic resin in a thickness direction of the base 7. The pin insert 5 includes three connector pins 11, 12 and 13 which are integrally mounted to the base 7. Since the connector pins 11 and 12 are aligned in a depth direction, they are depicted as one connector pin in FIG. 6.
The base 7 has a cylindrical external shape and is fitted to the connector sleeve 6 along the inner periphery of the connector sleeve 6. A threaded hole is formed in the base 7. The screw 9 is screwed into the threaded hole from the outer peripheral surface of the base 7 towards the center. The screw 9 is radially projected from the outer peripheral surface of the base 7. A shoulder of screw 9 is in contact with the inner peripheral surface of the connector sleeve 6. The outer peripheral surface of the base 7 at the opposite side of the screw 9 presses the inner peripheral surface of the connector sleeve 6 by screwing the screw 9 into the threaded hole. Thus, the pin insert 5 is fixed in the connector sleeve 6.
The connector sleeve 6 is fixed at a predetermined position in the microphone case 1 with a screw 10 screwed from the outer peripheral surface of the microphone case 1. Details about the fixing mode of the connector sleeve 6 to the microphone case 1 will be described below.
Since an electronic circuit including a semiconductor device is incorporated in the circuit board 3, penetration of electromagnetic waves into the microphone case 1 causes mixing of a noise into acoustic signals output from the microphone unit 2. In order to prevent such penetration of electromagnetic waves into the microphone case 1, an electromagnetic shield is effectively applied to the microphone unit 2. At the rear end of the condenser microphone (adjacent to the microphone connector) has an opening through which the electromagnetic waves disadvantageously penetrate. Accordingly, the electromagnetic shield is effectively applied by putting a shielding material on the rear end of the microphone.
As shown in FIG. 7, however, the condenser microphone is assembled by inserting a unit (FIG. 7A) that is composed of the circuit board 3 and the microphone connector 4 housed in the connector sleeve 6 into the microphone case 1 (FIG. 7B) through the opening (upper end of the microphone case 1), and fixing this unit to the microphone case 1 with the screw 10 screwed from the outer peripheral surface of the microphone case 1. Accordingly, the opening of the microphone case 1 cannot be completely sealed.
Furthermore, in order to insert the unit including the microphone connector 4 and the circuit board 3 into the microphone case 1, the outer periphery of the connector sleeve 6 needs to be slightly smaller in diameter than the inner periphery of the microphone case 1.
Accordingly, a gap between the connector sleeve 6 and the microphone case 1 is inevitably formed.
Next, the relationship between the inner peripheral surface of the microphone case 1 and the outer peripheral surface of the connector sleeve 6 will be described below with reference to FIG. 8, which is an enlarged cross sectional view of B-B′line shown in FIG. 6. A gap 8 is the difference between two diameters of the outer periphery of the connector sleeve 6 and the inner periphery of the microphone case 1. As explained above, the dimension of the gap 8 need to have a width enough to insert the connector sleeve 6 into the microphone case 1. The gap 8 shown in FIG. 8 is depicted larger than its real size.
A threaded hole is formed on the outer peripheral surface of the connector sleeve 6. A hole is formed on the outer peripheral surface of microphone case 1 at the position where corresponds to the threaded hole of the connector sleeve 6. The screw 10 is inserted into the hole of the microphone case 1 and screwed into the threaded hole of the connector sleeve 6, thereby fixing the connector sleeve 6 to the predetermined position of the microphone unit 2. Generally, about three screws 10 are used as shown in FIG. 8.
In a part fixed with the screw 10 and a certain area surrounding the fixed part, since the microphone case 1 and connector sleeve 6 attract each other with the screw 10, the outer peripheral surface of the connector sleeve 6 is distorted outward while the inner peripheral surface of the microphone case 1 is distorted inward. Accordingly, the gap 8 at which these distortions are created becomes smaller in width. Moreover, the connector sleeve 6 and microphone case 1 are electrically conducted through the screw 10.
The narrow portion of the gap 8 and the connector sleeve 6 and microphone case 1 electrically conducted through the screw 10 have an electromagnetic shielding effect against electromagnetic waves from the exterior. Since the electromagnetic shielding effect, however, cannot be obtained in the other area, the electromagnetic waves can enter the inside of the microphone case 1 through the gap 8.
In brief, the conventional microphone has an area through which the electromagnetic waves enter the microphone case 1 in the gap 8 between microphone case 1 and the connector sleeve 6. This causes noise in the condenser microphone.
For the purpose of preventing the electromagnetic waves from entering the inside of the condenser microphone, condenser microphones which include a microphone connector in which a shielding case covers one end surface of the base 7 in the pin insert 5, and the connector pin 11 for grounding is soldered to the shielding case to be electrically connected are known (disclosed, for example, in Japanese Unexamined Patent Application Publication Nos. 2005-094575 and 2005-311752).
Each of the condenser microphones disclosed in Japanese Unexamined Patent Application Publication Nos. 2005-094575 and 2005-311752 includes the microphone connector that prevents factors generating noise from entering the inside of the microphone case. These condenser microphones, however, do not have the shielding effect for preventing the electromagnetic waves from entering the microphone case through the gap between the connector sleeve and the microphone case. Accordingly, these condenser microphones do not have enough function of the electromagnetic shield that prevents electromagnetic waves entering from the opening of the microphone case.